1. Field of the Invention
The present invention relates to an improved strongback and method for suspending the strongback in a pattern mold and ultimately in an investment mold for casting mating articles.
2. Description of the Prior Art
Copending patent application Ser. No. 499,227, now U.S. Pat. No. 3,981,344 entitled "Investment Casting Mold and Process" and assigned to the assignee of the present invention describes a method for making a cast product, such as a hollow gas turbine blade, wherein the product is cast in opposed, mating halves on opposite sides of a central mold element suspended rigidly in the mold cavity of an investment shell mold. The central mold element utilized in the disclosed process is referred to as a strongback and includes article-forming surfaces on opposite sides thereof and edge flanges extending beyond these surfaces to be embedded in the shell mold. In making turbine blades and the like in accordance with the disclosed technique, the strongback is provided with bonding locators near the opposite ends of the article-forming surfaces. The locators may be in the form of a detent and mating projection on opposite sides of the strongback and are used to effect precision alignment of the blade halves for subsequent bonding. Such bonding locators are illustrated in the Hayes and Phipps patent, U.S. Pat. No. 3,965,963, with respect to a somewhat different casting process. As shown but not discussed in the patent, these locators are substantially colinear with the so-called design datum line of the turbine blade and strongback, which line is widely used as the reference line from which the blade and strongback structural features are measured and positioned.
In the casting process disclosed in the above-mentioned patent application, a critical step involves the attachment of the wax patterns of the article halves to the opposite sides of the strongback. If the patterns are not attached in precise dimensional relationship to the strongback and to one another, the investment shell mold formed around the assembly will have a strongback incorrectly suspended therein, thus producing misoriented blade halves when molten metal is solidified therein. The most commonly used technique for pattern attachment involves suspending the strongback in a pattern mold and injecting molten wax therein to form the patterns directly on each article-forming surface of the strongback. In this technique, locating means, such as knife edges, tabs and the like, are provided in the pattern mold to engage the outer edge periphery or perimeter of the strongback and locate it in proper relation in the mold. However, this technique introduces error in positioning the strongback in several ways. First, the outer edge perimeter of the strongback is made within a definable tolerance range, such as .+-. 0.005 inch, and therefore varies in dimension from one strongback to the next. Location of the strongbacks with reference to such a variable outer perimeter thereby results in positional variations of the strongbacks, and bonding locators thereon, in the pattern mold. Second, the ceramic or refractory material from which the strongback is made exhibits shrinkage during various stages of the process. When the strongback is positioned with reference to its outer perimeter, the amount of shrinkage between the design datum line with which the bonding locators are substantially colinear and the outer perimeter is oftentimes significant and introduces another source for strongback positioning variations. Third, the locating means provided in the pattern mold for engagement with the outer perimeter must have dimensional and positional tolerances to accept the variable strongback perimeter. As a result, some strongbacks are not held as rigidly as others and have been known to shift position in the mold when the molten wax is injected therein under pressure. Of course, these strongback positioning errors lead to misoriented wax patterns and eventually to the production of investment molds having the strongback imprecisely suspended therein. Articles cast in such investment molds may exhibit significant dimensional variations, such as in the thickness of the turbine blade wall, which are cause for rejection of the casting. An equally detrimental result is that article halves cast in different investment molds cannot be satisfactorily mated and bonded together as a result of the imprecise dimensional relationship therebetween. In the mass production of articles, such as turbine blades, it is highly desirable, if not imperative, that turbine blade halves cast in different investment molds be capable of subsequent mating and bonding to one another to produce the finished blade.
Copending U.S. patent application Ser. No. 722,181 entitled "Method for Positioning A Strongback" of Robert A. Herold has a common assignee with the present invention and provides a method for the improved positioning of a strongback in a pattern mold for formation of expendable, article-shaped patterns thereon. Improved positioning of the strongback in the pattern mold is achieved by suspending the strongback from the bonding locators by expendable locating means; that is, locating means which are incorporated into the patterns formed on the strongback surfaces. The use of expendable locating means enables the bonding locators to also serve as locators for positioning the strongback in the pattern mold and essentially eliminates dimensional and other tolerances associated with the prior art technique of outer perimeter location.